Discussion on the vacuum degree of sandwich in cryogenic liquid storage containers
Core Tips: Discussion on the Vacuum Level of Interlayer Laminated Containers Used in Cryogenic Storage Jiang Jilin1, Chen Haipeng2 (1. Jiangxi Oxygen Generator Plant, No.101 Lianhua Road, Lanshan District, Jiujiang City, Jiangxi Province, China; 332008; 2. Boiler Pressure Vessel in Jiangxi Province Inspection Institute, Jiangda South Road, Nanchang City, Jiangxi Province 330029, China)

Discussion on the Vacuum Level of Interlayers in Containers Used for Storage and Transportation at Low Temperature Jiang Jilin1, Chen Haipeng2(1.Jiangxi Oxygenerator Factory, No.101 Lianhua Road, Lanshan District, Jiujiang City, Jiangxi Province, China;332008;2. Inspection of Boiler and Pressure Vessels in Jiangxi Province Institute, Jiangda South Road, Nanchang 330029, Jiangxi Province, China. "Integration to use" in the principle of "conforming to the use of" in the reasonable determination of the regular inspection of the laminar vacuum in the eligibility criteria and ensure the safe use of cryogenic vessels have been analyzed and discussed. It is pointed out that the sealing vacuum degree given by the manufacturing standard is regarded as an eligibility criterion for the inspection, and the rate of reduction of the vacuum degree of each cryogenic container is different, which leads to different criteria for determining the degree of vacuum. ), Male, Senior Engineer, graduated from Wuxi Institute of Light Industry in 1970, worked in Jiangxi Oxygen Making Machine Factory in 1971, and now serves as Chief Engineer of Jiangxi Oxygen Making Machine Factory.

With the development of our country's economy, the use of vacuum-powder adiabatic cryogenic vessels has become more widespread, the establishment of large numbers of cryogenic liquid storage stations, and the deployment of large numbers of cryogenic liquid carriers, and the corresponding safety hazards have been increasing. Therefore, it is imperative to carry out reliable periodic inspections of these storage containers to ensure their safe operation. The current situation in China is as follows: 1 Due to the equipment and testing technology, there are few units that can carry out such periodic inspections. The units that have carried out regular inspections are also a matter of recent years. The time is not long; 2 new pressure vessel safety technology inspection The regulations stipulate some basic inspection items, but there are no uniform standards and specifications for the specific inspection contents, inspection methods and qualification standards. In other words, the inspection rules for cryogenic vessels are not yet mature, and they need to be continuously explored and summarized. , improve the detection level and the reliability of the test.

As the structure of cryogenic vessels mostly belongs to sandwich vacuum insulation, the performance of the product when used, ie the evaporation rate, depends on the degree of vacuum in the sandwich. Therefore, for a cryogenic vessel in use, the degree of vacuum in the sandwich is a very important indicator for ensuring the safe operation of the product. For a cryogenic vessel that has been in operation for a period of time, the degree of vacuum in the sandwich has changed from that at the time of completion of manufacture and exceeds the degree of sealing vacuum specified in the manufacturing standards. Therefore, the determination of the vacuum level eligibility criteria in regular inspections is particularly important. It is necessary to conduct research and discussion on the topics.

1 The basic conditions of the vacuum determination criteria In order to solve the criteria for determining the degree of vacuum in a cryogenic vessel, it is necessary to understand the influence of manufacturing conditions and the vacuum vacuum adiabatic cryogenic vessel sandwich vacuum and its effect on the adiabatic performance.

1.1 Overview of manufacturing standards for cryogenic storage containers For the manufacture of cryogenic storage containers, China's manufacturing standards are divided into two phases: the first phase, prior to 2000, the use of mechanical industry standards, standards stipulates: during the design of cryogenic vessels, While considering the degree of vacuum, it is necessary to determine the leak rate; the standard specifies the standard for sealing vacuum degree, but no given leak rate; therefore, before 2000, when designing a cryogenic vessel, it is necessary to determine the specific value of the leak rate. In general, the leak rate is determined based on the quality assurance period of the product (18 months). The cryogenic vessels currently in use are all designed and manufactured using the standard of this stage. In the second stage, in 2001, the National Standard for Cryogenic Adiabatic Pressure Vessels organized by the State Bureau of Quality and Technical Supervision, in order to ensure the performance of the products, directly The total vacuum leakage rate of the cryogenic vessel, the gas exchange rate of the vacuum interlayer, and the degree of vacuum (vacuum vacuum) that the interlayer should achieve during vacuum sealing are manufactured. This means that, in the design of cryogenic vessels, it is not necessary to calculate the gas leakage rate of the interlayer, but only the values ​​given in the standard, so that the design criteria of the product are unified and standardized.

1.2 Influencing factors of interlayer vacuum degree and its effect on the thermal insulation effect 1.21 - Generally speaking, there are three main influencing factors for interlayer vacuum degree and thermal insulation performance: Influence of powder (pearlescent sand) in interlayer on thermal insulation performance: due to the physical properties of the powder and Different physical shapes, its thermal conductivity is also different. In addition, the same powder, when in different vacuum degrees, its thermal conductivity is not the same, the basic law is that as the degree of vacuum decreases, the powder thermal conductivity increases.

The effect of gas in a vacuum sandwich on insulation performance: As the vacuum level decreases, the gas content in the vacuum sandwich increases, and the thermal conductivity necessarily increases.

(3) The effect of deflation of various materials in the sandwich on the degree of vacuum: With time, the deflation of the material is unavoidable, which will lead to a reduction in the degree of vacuum. In addition, leaks in the interlayer and a drop in the degree of vacuum will lead to deterioration of the thermal insulation performance.

1.22 The degree of vacuum in the sandwich reduces, resulting in the failure of the insulation effect of the sandwich. There are currently no specific standards, but there are two kinds of understanding: several factors that lead to a reduction in the degree of vacuum, the industry standard before 2000 considered that when the degree of vacuum in the sandwich is reduced to At 65Pa, the insulation performance of the sandwich will drop dramatically, leading to adiabatic failure, such as.

Relation between vacuum degree and heat insulation performance of 65Pa vacuum degree sandwich (2) Vacuum powder heat insulation low temperature container, after filling cryogenic liquid, when the vacuum degree of interlayer is better than SXKPa, the gas thermal conductivity is also lower than that of powder; when the vacuum degree is reduced to After 1Pa, the effect of gas heat transfer may multiply the evaporation rate and lose the use value of the cryogenic container. Such as.

Vacuum degree after liquid filling Vacuum powder Insulation of vacuum powder The relationship between the degree of vacuum after filling the liquid and the thermal conductivity of the air According to the above, given that the cryogenic container is a dangerous product under the national mandatory management, we believe that in the use of cryogenic containers at regular intervals The degree of vacuum at the time of inspection shall be set at 10 Pa. 2 Determination of Vacuum Rating Criterion 2.1. Guiding Principles For the use of cryogenic vessels, there is a guiding principle in the use of pressure vessel inspection procedures - the principle of "fit for use". According to this principle, the criterion for determining the degree of vacuum in a cryogenic vessel should be: ensuring the safe use of the product until the next inspection cycle. The B/T 9072 fixed liquid powder insulated cryogenic storage tank requires a one-year inspection period of one year. After one year, the vacuum level of the cryogenic vessel sandwich is less than 10 Pa. 2 The factory-sealing vacuum degree is regarded as the “fixed inspection” standard. It is reasonable to think that the standard seal vacuum degree given by the standard should be used as the criterion for “fixed inspection” because the leakage rate has been given in consideration of the design. Therefore, using the factory vacuum level can guarantee the safe use of the product to the next inspection cycle. This inference seems reasonable, but from the actual situation, this method is not reasonable, because: The leak rate determined at design time is an upper limit value. The actual leakage rate of most products is lower than this value. Because the manufacturing process and environment (including process parameters, operating levels, environmental conditions, etc.) are somewhat different in the manufacture of each cryogenic vessel, the actual leakage rate of each cryogenic vessel varies, reflecting the actual use of cryogenic vessels. We can find that some cryogenic vessels manufactured in 1995 and 1996 have so far had a good thermal insulation effect, and some cryogenic vessels need to be re-vacuumed after one or two years. In other words, the actual warranty period for the vacuum level of the cryogenic vessel is not the same as the design guarantee period and may be longer.

The vacuum degree of the vacuum insulation interlayer of the cryogenic storage vessel cannot be maintained after leaving the factory, even if it is an absolutely sealed sandwich. Due to the “exhalation” phenomenon of the material itself, the vacuum level cannot be guaranteed. If the vacuum level specified in the manufacturing standard is used as the criterion for the annual inspection, each cryogenic container will be vacuumed once a year. For cryogenic vessels with good thermal insulation properties In fact, this dogmatic practice violates the "use for use" in the pressure vessel inspection procedure.

Principle, but also causing waste.

3 Determination of the degree of vacuum determination criteria and vacuum degree calculation Through the above analysis, and from the foregoing guidelines, in order to ensure the safe use of cryogenic containers to the next test cycle, we believe that the idea of ​​determining the vacuum degree determination criteria should be as follows: First, each The qualification criteria for the degree of vacuum of the cryogenic vessel are not the same and it depends on the rate of decrease of the vacuum of the interlayer. The concept of “rate reduction rate of vacuum” was introduced mainly because this concept comprehensively considers leakage of interlayers and deflation of materials in the interlayer. Therefore, it is necessary to determine whether the vacuum degree of each product sandwich is qualified through calculation. The process is as follows: Calculate the rate of vacuum reduction P of the cryogenic container sandwich, the actual measurement of the vacuum degree of the sandwiched product at the time of the final inspection, Pa 0 product last time The actual measurement of the sandwich vacuum during the inspection, the Pa geometry volume (design given or actual measurement), the time of the last calibration of the Lt product to the scheduled inspection Note: The premise of this calculation is to assume that the rate of vacuum reduction does not change over time. Changes, this assumption can be corrected by multiple actual calculations.

Calculated to the next! The degree of vacuum that may be achieved during the test cycle is the conversion formula (1) from t to the time of the next test cycle (one year) The measured vacuum of the sandwich, Pa P, vacuum at the next test cycle, Pa geometric volume, L Compare, if Pt

P rules need to be vacuumed again. The use of the degree of vacuum specified in the operating instructions as a criterion is due to the different design methods and considerations at the time of design of each plant. As a principle of regular inspections, it is necessary to respect the requirements of designers, in accordance with national regulations and standards. At present, in the absence of a vacuum standard in the country, the given value of the design specification should be used as a basis. However, in view of the foregoing discussion, we believe that if the design is not explicitly given or the given value is high, 10Pa should be used as the basis.

3 Conclusion Due to the special structure of the cryogenic storage container product structure, the sandwich vacuum degree has become a very important indicator to ensure the safe operation of the product, and the determination of the vacuum standard for the sandwich temperature in the cryogenic vessel has a certain degree of complexity. The principle of “combined with use” has been analyzed and discussed in order to reasonably determine the eligibility criteria for laminar vacuum in periodic inspections and to ensure the safe use of cryogenic vessels. It is summarized as follows: When the sealing vacuum given by the manufacturing standard is used as the inspection The eligibility criteria are unreasonable.

The rate of vacuum reduction at each cryogenic vessel is different, resulting in different criteria for the degree of vacuum.

Therefore, the determination of the vacuum degree of each cryogenic vessel needs to be calculated and a corresponding calculation and determination method is proposed.

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